Patau Syndrome—Trisomy 13
Trisomy 13, also known as Patau syndrome is a chromosomal condition in which there are three copies of chromosome 13 in the cells of the body instead of the normal two. Trisomy 13 occurs in about one in 10 000 live births and is associated with approximately one per cent of all miscarriages.
Characteristics of Trisomy 13
The term syndrome is used to describe a collection of distinctive characteristics that are often seen together in an affected individual. Babies with Trisomy 13 have recognisable characteristics referred to as Patau syndrome. Dr Klaus Patau first described the specific features associated with this chromosomal abnormality.
Babies with Patau syndrome are likely to have many of the following characteristics (but not necessarily all of them):
- Severe feeding difficulties with poor growth and weight gain.
- Poor muscle tone and episodes when spontaneous breathing may temporarily stop.
- A small head with a sloping forehead.
- Facial features which may include widely set small eyes with vertical skin folds covering the eye's inner corners, a broad flat nose, low-set ears and a cleft lip and palate.
- Hand and feet abnormalities including extra fingers and toes, and changes to the heel giving them a sometimes 'rocker bottom' shaped foot.
- Incomplete development of certain parts of the brain.
- Undescended testes in boys.
- Kidney and heart defects, which are often severe.
- Severe developmental delay and intellectual disabilities.
These characteristics are common for babies with Trisomy 13, but it is important to remember that each baby will be different with their own unique appearance.
Causes of Trisomy 13
Trisomy 13 is caused by an extra copy of the chromosome number 13. The chance of having a baby with Trisomy 13 increases with maternal age. It also increases slightly if the mother has had a previously affected pregnancy. It is estimated that every three to four fertilised eggs, out of ten, have a chromosomal problem and this rate increases with the mother's age. The majority of these conceptions miscarry early, often before they have even been recognised as a pregnancy.
Short life expectancy
Sadly, the prognosis for babies with Trisomy 13 is very poor and the majority of these babies are stillborn Some babies will survive pregnancy and labour but few of these will survive their first month of life. It is very rare for a baby with Trisomy 13 to survive the first year of life.
Normally egg and sperm cells have 23 chromosomes each, which join to form a future baby with 46 chromosomes (23 plus 23) in each cell. During fertilisation each of the chromosomes from the egg and the sperm fuse and separate. If this separation does not occur properly, a cell with three copies of the chromosome is the result. This cell then starts rapidly copying itself to eventually form a baby with each of its cells having the extra copy of chromosome 13. It is this extra chromosome that causes the intellectual and physical characteristics of Trisomy 13.
Mosaicism and translocation
In occasional cases of Trisomy 13 the extra copy of the chromosome 13 is not in every cell. This situation is called Mosaicism (mixture) and is likely to result in less severe characteristics of the syndrome.
Very rarely, either of the parents are carriers of a copy of the chromosome attached to a different chromosome. This is called a translocation (rearrangement) and may not affect the parents. However future pregnancies are more likely to be affected when further rearrangements of the chromosomes may occur. In these instances genetic counselling is recommended.
Antenatal screening tests for Trisomy 13 have been developed and are performed in conjunction with Down syndrome screening tests. These tests include the first trimester combined screening test (involving the nuchal translucency ultrasound measurement and blood test between 11+0 and 13 +6 weeks), the second trimester triple test (blood test) and obstetric ultrasound between 18 and 20 weeks. Approximately 90 per cent of affected babies are detected at screening.
Trisomy 13 can be suspected prenatally (before birth) or after birth; however the diagnosis can only be made after chromosomal analysis. Diagnosis can be confirmed prenatally by chromosomal analysis on samples obtained following ultrasound-guided procedures such as chorionic villus sampling (CVS) or amniocentesis. For further information on prenatal testing please refer to Mater Mothers' Hospital's brochure: Prenatal diagnostic procedures.
Trisomy 13 is a chromosomal abnormality which cannot be treated or prevented. If born alive, admission to the Neonatal Critical Care Unit is usually necessary for a baby to survive after birth.
Some parents may instead elect perinatal palliative care for their baby, keeping their baby with them or nearby until they pass away.
All families with an affected baby will be offered counselling by a neonatologist or baby doctor regarding the management of their baby.
There is approximately a one per cent chance of conceiving a baby with Trisomy 13 in a future pregnancy. This percentage progressively increases for mothers over the age of 35.
Genetic Health Queensland (GHQ) provides a genetic counselling service three days a week at Mater's Centre for Maternal Fetal Medicine. A genetic counsellor is available to discuss all aspects of screening and diagnostic testing for Trisomy 13. To speak to a genetic counsellor please telephone 07 3163 1593.
For further information, we recommend you speak to your doctor. Alternatively, you may find the following websites useful:
Centre for Maternal Fetal Medicine
Mater Mothers' Hospital
South Brisbane Qld 4101
Phone: 07 3163 1896
Fax: 07 3163 1890
Mater acknowledges consumer consultation in the development of this patient information.
Last modified 09/11/2015.